1. Field of the Invention
This invention relates to a spindle motor adapted to rotate a data record disk (hereinafter referred to merely as "a disk", when applicable) such as CD, DVD, CD-ROM and DVD-ROM, and more particularly to a spindle motor which suppresses the swing or vibration of the shaft which attributes to the eccentric gravity center during rotation.
2. Related Art
In general, in a spindle motor adapted to turn a disk, a turn table is fitted on one end of the shaft, and a disk is mounted on the turn table. The turn table has a circular-truncated-cone-shaped or semi-spherical protrusion at the center, while the disk has a central hole. With the protrusion engaged with the central hole of the disk, the central position is determined.
When the turn table is turned by the spindle motor, the disk is also turned together with the turn table, so that the signals recorded on the recording track of the disk are read by a reading section such as an optical pickup. In the case of a rewritable or writable disk, data signals can be rewritten or written in the disk. As in the case of a recent CD-ROM drive unit, in order to increase the data reading and writing speed there is a tendency to increase the speed of rotation of the disk. Accordingly, there has been a strong demand for the provision of a spindle motor which can turn the disk at higher speed.
However, if the speed of rotation of the disk is increased, then the unbalance of the centrifugal force is increased which occurs during the rotation of the disk, as a result of which the shaft is rotated while swinging. When the shaft turns while swinging, and its vibration is increased, then it is difficult for the reading section to accurately read data signals recorded on the disk, or it is difficult for the writing section to accurately write data signals on the disk.
In general, the centrifugal force attributing to the eccentric gravity center of a disk is proportional to the square of the speed of rotation, and as the speed of rotation increases, the swinging and vibrating of the shaft is extremely increased. Accordingly, the swinging and vibrating of the shaft, which attribute to the eccentric gravity center, are serious problems when it is required to increased the speed of rotation of the disk.